Removal of trivalent chromium in the complex state of trivalent chromium passivation wastewater

• A novel two-step technology for the trivalent chromium complex is proposed.
• The trivalent chromium complex is decomplexed via oxidation to hexavalent chromium.
• The hexavalent chromium is reduced to trivalent chromium and precipitates as Cr(OH)3.
• Almost thorough Cr removal of real wastewater is achieved by two-step technology.

In this paper, a novel two-step technology was proposed to treat the stable complex trivalent chromium (Cr(III)) passivation wastewater based on the advanced oxidation processes (AOPs), such as ozonation (OZ) or three dimensional electrode (TDE), with a subsequent electrocoagulation (EC). The results show that the oxidation processes is fundamental for the removal of complex Cr(III). In the first step, the stable Cr(III) complex was completely decomplexed via oxidation to hexavalent chromium (Cr(VI)) by the radicals (OH) or oxidants (O3, H2O2) produced in OZ or TDE process. In the second step, the formative Cr(VI) was reduced back to dissociated Cr(III) in the electrogeneration of Fe(II), which precipitate as Cr(OH)3 with Fe(III) ions via hydroxide or polyhydroxide formation. Almost thorough Cr removals (lower than 0.03 mg L−1) of ten types of real complex passivation wastewater were successfully achieved by combining OZ or TDE with EC.